IL-2 is an essential cytokine that plays a central role in maintaining T cell homeostasis and mediating proper immune responses. Its high potency as an immune stimulator has led to clinical uses to treat a range of conditions, including cancers and AIDS; it is also widely used as an adjuvant for vaccination to stimulate activation and proliferation of various effector cells.
However, the high dose of IL-2 that is required for effective treatment of certain diseases is highly toxic. Major adverse effects of such therapy include vascular leak syndrome (VLS), which results in accumulation of the intravascular fluid in organs such as lung and liver with subsequent pulmonary edema and liver damage. There is no treatment for VLS except withdrawing the therapy.
IL-2 exerts its pleiotropic functions by binding to different combinations of receptor components expressed on different cell types: the alpha chain (IL-2Rα, also known as CD25), the beta chain (IL-2Rβ, or CD122), and the common cytokine receptor gamma chain (IL-2Rγ, γc, or CD132).
Isolated IL-2Rα has been termed the “low affinity” IL-2 receptor (binding affinity KD˜10 nM) and is not involved in signal transduction. A complex of IL-2Rβ and γc binds IL-2 with intermediate affinity (KD˜1 nM), although IL-2Rβ alone has very low affinity (KD˜100 nM) and γc alone has virtually no detectable binding affinity for IL-2. A complex with all three subunits, IL-2Rα, IL-2Rβ, and γc, binds IL-2 with high affinity (KD˜10 pM).
Heterodimerization of IL-2Rβ and γc is necessary and sufficient for effective signal transduction through the interaction of their cytoplasmic domains and subsequent kinase activation of multiple signaling pathways; IL-2Rα plays no role in signal transduction.
High-affinity α-β-γc IL-2Rs are typically found on CD4+ T regulatory cells (Tregs) as well as recently-activated T cells. Intermediate-affinity β-γc IL-2Rs are present at a low level on naïve CD8+ cells, but are prominent on antigen-experienced (memory) and memory-phenotype (MP) CD8+ T cells as well as natural killer (NK) cells. Both MPCD8+ T cells and NK cells express very high levels of IL-2Rβ and readily respond to IL-2.
Previous studies have indicated that VLS is caused by the release of proinflammatory cytokines from IL-2-activated NK cells. However, a recent study suggested that IL-2-induced pulmonary edema may result from direct binding of IL-2 to lung endothelial cells, which express functional high affinity α-β-γc IL-2Rs. This was evidenced by the observation that interaction of IL-2 with lung endothelial cells was abrogated by blocking anti-IL-2Rα monoclonal antibody (mAb), in IL-2Rα-deficient host mice, or by the use of an IL-2/anti-IL-2 mAb (IL-2/mAb) complex in which the antibody prevents IL-2/IL-2Rα interaction, thus preventing VLS.